1. Field of invention
This invention relates to measurement of liquid level, specifically to an apparatus employing multi-symbol refractive optical encoding to determine the absolute position of a float on a slender rodlike scale.
2. Background Description of Prior Art
U.S. Pat. No. 5,950,487 Maresca, et al. summarizes the prior state of the art. Maresca et al. describe a multiple-float, liquid-level gauging system that employs a flat, flexible, and wide measurement scale tape. Distance or level information is encoded on numerous parallel tracks as nominally opaque segments in an otherwise transparent scale tape. That is, elements of the scale tape either block or transmit visible or infrared light, or some other energy. Because only on-off binary data may be encoded on the Maresca et al. scale, space utilization on the scale is inefficient, and many tracks are needed to encode distance or level information. For example, 12 such tracks are needed to encode 4096 levels. More tracks and a wider tape are necessary to increase the number of levels. The Maresca et al., scale also requires a relatively large number of emitters and detectorsxe2x80x94one each for every scale track or encoded bit. Elaborate methods are described by Maresca et al. to hold the flexible scale in a vertical position, stretch it out, or to anchor it. The wide flat tape used for the Maresca et al. scale also requires special efforts to circumvent the tendency of the tape to stick to the inside of the wide slot in the float. The wide tape also means that the float is similarly large. As is typically the case with such systems, the Maresca et al. on-off encoding system is also subject to inadequate opacity, or light blocking. Pinholes and thin spots allow light to leak through, reducing contrast, and leading to errors. Maresca et al. also describe a three-layer tape scale necessitated by a requirement to print the opaque segments on an inner layer, which must be sandwiched between two other protective layers. A two-layer tape is simpler, easier, and cheaper to construct.
There are numerous examples where light or other energy is either blocked or transmitted to encode absolute distance or level information. Other examples involve various arrangements with freely moving floats and the use of Gray codes. For example, U.S. Pat. No. 5,483,831 Steiner provides an example of a battery-powered float on a vertical guide with optical reflective encoding. The Steiner float is off center from the guide, has a large contact area with a guide, and employs a cable for data transmission, all of which features tend to restrict float motion. Numerous tracks are needed to provide many unique distances or levels. U.S. Pat. No. 5,880,683 Brandestini describes a ternary (base 3) absolute Gray code digital position encoder that uses only a single track. Additional code elements in are inserted longitudinally in the single track. Consequently, resolution is degraded, and the number of positions encoded is limited. U.S. Pat. No. 5,574,445 Maresca et al. describes a multi-cycle interleaved Gray code scale that also provides a reduction of the number of tracks required. This type of encoder also employs binary on-off (transparent and opaque) state detectors. Yet others use transmissive or reflective elements to count relative changes in distance along a scale. U.S. Pat. No. 5,428,863 Biggel and U.S. Pat. No. 5,585,786 Clark et al. provide additional recant examples of this type of encoder.
U.S. Pat. No. 5,453,839 Samuelsson describes use of an optical diffuser to spread a light beam out over a subgrouping of photoelements in order to determine the centroid of the beam, and thus accurately measure its position to sub-beam precision. There is no distance encoding, and a long row of photoelements is needed to span a distance interval. Only short spans are practical.
U.S. Pat. No. 5,359,184 Froehlich et al. describes optical encoding utilizing selectively refracted light. Refractive encoding is used to identify cuvettes employed in automated testing of biological samples. Wedge-shaped beveled facets are molded into flanges on the cuvettes to encode the cuvette identity. Bevels in two opposite orientations selectively refract light onto one or the other of two closely-spaced detectors, whose outputs are compared to recover the binary code bit value. Manual adjustment is provided to account for detector sensitivity variation. Several (six, in the examples given) such facets provide a number (64, in the example) of distinct codes. Other objects besides cuvettes, having suitable places for transparent facets, are also said to be identifiable using this technique. However, the use of uniform single wedges across the facets limits usefulness. Thick facets are necessary to accommodate large wedge angles. Large wedge angles are necessary to produce useful lateral beam deviations over short ranges (Fresnel-lens-like facets with a serrated surfaces would be more effective). Single uniform wedges cannot split an incident beam into multiple beams, and thus increase the number of symbols encoded per facet. There is no arrangement of facets with progressive or sequential encoding of symbols with distance, which is necessary to form a measurement scale.
In accordance with the present invention a liquid level sensor comprises an autonomous float on the surface with an internal optical reader to decode distance or level information encoded in the dispositions of microrefractive facets on a slender vertical rod. The rod also conducts decoded level information to the rod support. An optical reader in the rod support provides a reference level.
Accordingly, the overall object of this invention is to provide an advanced method and apparatus for measuring liquid level. Specific objectives and advantages of this invention include reducing the number of encoding tracks and electronic components and the size, weight, and width of floats and of the level-encoding scale.
Another objective and advantage of this invention is to provide multi-symbol encoding per encoded level.
Another objective and advantage of this invention is to increase the number of encoded levels.
Other objectives and advantages of this invention are to reduce the possibility of measurement errors and to provide fault detection.
Another advantage of this invention is the use of an encoding rod scale that permits the decoder light beams to share space in the encoding scale.
Yet another objective and advantage of this invention is to employ the encoding scale as a signal conduit and to provide means for coupling signals into and out of the scale.
Another objective and advantage of this invention is to increase battery life by reducing power requirements.
A further objective and advantage of this invention is to simplify installation.
Yet another objective and advantage of this invention is to provide a reference level.
Still further objects and advantages will become apparent from a consideration of the ensuing description and accompanying drawings.